WASHINGTON, D.C. — Islamic State extremists have discussed infiltrating the United States through its border with Mexico, a U.S. official said Wednesday.

Francis Taylor, under secretary for intelligence and analysis at the U.S. Department of Homeland Security, told a Senate committee Wednesday that the Sunni militants have been tracked discussing the idea on social-media sites such as Twitter.

“There have been Twitter and social-media exchanges among ISIL adherents across the globe speaking about that as a possibility,” Taylor said. Islamic State is also known by the acronyms ISIL and ISIS. Referring to the 1,933-mile (3,110- kilometer) boundary with Mexico, Taylor said he was “satisfied that we have the intelligence and the capability at our border that would prevent that activity.”

Taylor’s comments before the Homeland Security and Governmental Affairs Committee came hours before President Barack Obama is to outline in a speech plans to expand the U.S. offensive against Islamic State. Steps under consideration include blocking foreign fighters from entering Syria and Iraq, delivering more aid to moderate factions among Syrian rebels, and expanding air strikes to Islamic State targets in Syria.

Taylor said the security of the U.S.’s southwest border remains a high concern for his department, and that Homeland Security Secretary Jeh Johnson has ordered “a comprehensive southern border security strategy that will include national security risks to border.”

Officials testifying before the committee said that Islamic State poses the greatest threat to U.S. interests in Iraq and within the region. At the moment, Islamic State’s ability to develop significant, large-scale attacks diminishes with distance from Syria and Iraq, said Nicholas Rasmussen, deputy director of the National Counterterrorism Center.

“We do not assess right now they have the capability to mount an effective large-scale attack on the United States,” Rasmussen said.

Islamic State’s sweep across Iraq and a campaign of terror that has included the beheading of two U.S. journalists have galvanized fears among U.S. citizens of a rising terrorist threat and stirred demands from lawmakers that Obama articulate a plan for dealing with the issue.

A Border Patrol vehicle is driven along the U.S.-Mexico border fence as agents carry out "special operations" on July 30, 2009 near the rural town of Campo, some 60 miles east of San Diego, California. David McNew/Getty Images/AFP

Sixty-five percent of U.S. citizens back bombing strikes against the extremists in Syria, more than double the level of support from a year ago, according to an ABC News/Washington Post poll released yesterday.

Islamic State members have surfaced in Europe — specifically in a shooting that killed four people at a Jewish museum in Brussels and through arrests in Paris — “a clear indication of ISIL’s ambition to operate outside the Middle East,” Rasmussen said. If the group is left to grow, that threat will only increase, he said.

“Left unchecked, ISIL poses a threat to all governments it considers apostate,” Rasmussen said, adding that the group’s targets would include governments in Europe, the U.S., Africa and the Middle East.

Lawmakers concentrated on the threat posed by foreign fighters who join Islamic State, either Americans or Europeans who wouldn’t need a visa to enter the United States.

Rasmussen estimated that “over 100 persons from a wide variety of backgrounds have attempted to travel to the region” from the U.S. to fight alongside extremist groups active in Iraq and Syria.

Many go “simply looking to join the fight” and engage with extremist elements, not necessarily searching to join Islamic State in particular, Rasmussen said. “Where they end up actually affiliating plays out over time,” he said.

Rasmussen said the estimate of 100 people included individuals who show intent to travel but haven’t left the U.S. as well as those who remain there, have been killed and who have returned.

One wild card will be U.S. citizens who may be radicalized through the Internet and decide to take up arms here, Rasmussen said.

If there is any good news, Rasmussen said, it is the “aggressive information-sharing with all of our partners who have the same problem.” This has given the U.S. and Europe “a significant leg up” in attempts “to disrupt travel when these individuals attempt to leave Iraq,” he said.

Gathering data on Islamic State and other extremists groups has become more difficult since disclosures by former National Security Agency contractor Edward Snowden, Rasmussen said, as intelligence agencies have seen terrorists change their methods and means of communication to avoid detection.

Those changes “frustrate our counterterrorism efforts,” he said. “We cannot connect the dots if we cannot collect the dots.”